In previous studies, we demonstrated, using radiation chimeras, that there exists a nondeletional mechanism for inducing self tolerance in the thymus. In attempts to establish the cell type responsible for this induction, it was shown that the thymic epithelium is unable to synthesize the Mls self antigen. Other experiments indicated that both CD4+ and CD8+ thymocytes are anergized in the chimeras for the class II-dependent antigens under study. While these data are compatible with energy induction occurring at a CD4+8+ stage, anti CD4 treatments fail to affect the energy displayed by CD4-8+ thymocytes. TCR alpha beta transgenic mice have been obtained and bred onto different MHC backgrounds. This more homogeneous systems should facilitate our future studies of thymic anergy induction. Further work on CD4+8- thymocytes indicates that the functional differences observed in the Qa-2+ and Qa-2- subsets may be related to additional requirements for accessory and/or adherence molecules for T cell activation. Studies are in progress to establish whether these observations can be related to anergy induction in these in thymocyte subsets. Double transgenic mice, transgenic for CD8.1 and TCR alpha beta (anti-HY, H-2D b-restricted) have been constructed to establish the mechanism for coordinating accessory molecule usage and MHC class specificity and to determine the stage at which positive selection occurs. The experiment was designed such that the CD8.1 transgene is constitutively expressed throughout development. The results obtained are consistent with an instructional model where the TCR-MHC interaction involves co-engagement of the appropriate accessory molecule at the CD4+8+ precursor stage. This event instructs the turn off of the other accessory molecule. The data are inconsistent with a selection model in which there is a stochastic down-regulation of one of the accessory molecules followed by TCR-MHC selection. Other studies of a CD4 transgenic line which expresses CD4 only on a proportion of peripheral CD8+ T cells, and not in the thymus, provided evidence that the accessory molecules determine MHC class-specific allorecognition.